This invention relates to an optical reader and, in particular to an improved optical reader which is relatively unaffected by "specular" light; where the effect of "specular" light arises when a light beam is projected on a mirror like surface and is reflected back.
Known optical reading devices are designed to project light onto an object containing information and to read or sense the light reflected from the object. The object may be any one of a large number of products and the information may be any type of information such as, for example, a bar code or alphanumeric information. Typically, the reader includes a light source and means for projecting light from the light source onto the object and light reflected from the object is optically imaged onto an image sensor contained within the reader; where the image sensor is designed to convert the reflected light into electrical analog signals.
A problem with the prior art may be best illustrated by reference to FIGS. 1 through 4. FIG. 1 shows a light source 11 whose light output is projected via a radiating lens 12 onto an object 13 containing information to be read. The term "radiating" or "irradiating" lens as used herein refers generally to a cylindrical lens for projecting the light from the light source onto an object. Light reflected from the object 13 is projected through an aperture stop and via an imaging lens 15 onto an image sensor 17. The image sensor 17 may include a photodiode array coupled to a charge coupled device (CCD) which functions as a photodetector and is an optical to electrical analog signal converter. In FIG. 1, the reflected image projected onto the image sensor 17 is in response to "scattered" reflective light and not due to "specular" reflective light; "specular" light is light reflection from a mirror like surface. An information pattern read in response to "specular" light reflected onto image sensor 17 is not a correct indication (reflection) of the information pattern contained on the object.
However, known optical readers such as bar code readers can receive strong specular light reflected from an object being read via the optical path between the object 13 and the image sensor 17 as shown in FIG. 2. This is illustrated in FIG. 2 where the light 14 is incident on object 13 at an angle .THETA..sub.2 such that it produces a reflected light beam 16 which represents a mirror like reflection denoted as specular reflective light. The reflection of specular light depends on the angle position at which the reading device is held relative to the object 13 which contains information to be read. FIG. 3 shows a top view of light beams incident on an object and reflected back onto the image sensor 17.
It is evident that the direction of the specular light reflected from an object varies as a function of the angle of the illuminating light incident on the information material. Therefore depending upon the angle of the incident illuminating light, strong specular light may be reflected along the optical light path imaged onto the image sensor. A high degree of "specular" reflective light imaged onto the image sensor significantly interferes and/or prevents the reading of the "correct" information available from the "scattered" light reflected back onto the image sensor.
The reflection of "specular" light occurs when the illuminating lens and the aperture stop coincide with two focus points of an ellipse and the reflective point coincides with the tangential line at a point of the ellipse.
Therefore, light incident on an object over a certain range of angles and reflected from the object with mirror like qualities onto the image sensor results in incorrect readings. The range of angles over which incorrect readings are obtained due to "specular" reflection is called the "dead zone". The "dead zone" depends on the angle position at which the reading device is held relative to the information pattern. FIG. 4 shows a bar code. Waveform B of FIG. 4 shows the waveforms produces at the output of an image sensor 17 when "scattered" light is reflected to the sensor. Waveform C in FIG. 4 illustrates the problem when the reader is reading the bar code A and the angle of the reader relative to the bar code being read is such that "specular" light is reflected back. Note that as shown in waveform C the information from two bars of the code are lost. This is a significant problem which exists with known optical readers and with known bar code scanners.
In many optical readers, several light emitting diodes (LEDs) and a radiating lens are used in an attempt to project uniform illumination onto the object or material to be read. The several LEDs are aligned or arranged in a row and, each LED functions as a point source of light. However, if one of the LEDs is not functioning or the light from that LED is not properly projected onto the object, the illumination will be uneven and non-uniform. When an optical information pattern such as bar code or an alphanumeric character recorded on a label or other material, is illuminated non-uniformly, the optical information will not be read accurately. This is another problem associated with known optical readers.
Furthermore, there exists non-uniform light energy passing through the aperture and imaging optics which is another problem present with known optical readers.
An object of this invention is to eliminate specular reflective light in the light path between the information material to be read and the image sensor located in the information reading device.
Another object of the invention is to compensate for the uneven and/or non-uniform illumination of the information material to be read.
A still further object of the invention is to compensate for the non-uniform light energy passing through the aperture and imaging optics in the information reading device.